Adds five tests covering the embedded dashboard flow against the
world_health example: render, hideTitle UI config, chart rendering,
allowed_domains referrer check, and guest-token data access. Includes:
- A chromium-embedded Playwright project, excluded from the main
project via testIgnore so it can be opted into separately.
- An EmbeddedPage page object and API helpers for embedding/guest
tokens plus dashboard lookup by slug.
- A static test app (embedded-app/index.html) loaded from a minimal
Node static server. Playwright bridges the guest-token fetch from
Node into the browser via page.exposeFunction.
- EMBEDDED timeout/config constants.
Workflow integration and test-environment configuration land in a
follow-up commit.
